Try to give them facts and lead them to articles. If you become overly emotional in your response (I know it's hard !!!), they will discount you as a raving cat person.
Many people are not aware of what actually takes place in a declaw surgery. But remember that your advice will be more easily accepted, if you demonstrate a straight forward answer backed up with declaw options and articles. I'm just sickened when I see vets so easily offer the declaw option to their clients. It's heartbreaking when there are alternatives.
Remember your there to educate, not to flame. Good luck!